Electrophoretic coating process



Aug. 10, 1965 BURNSIDE EIAL 3,200,057

ELECTROPHORETIC COATING PROCESS Filed Dec. 27, 1960 2 Sheets-Sheet 1POWER SOURCE I- 1 I7 I II II I II II I POWER SOURCE POWER SOURCE I I I II 11/' E El? REWER 5%7 L. UR/VS/DE F/G' 2 INVENTORS BY efiazv 10,1955 6.1.. BURNSIDE ET'AL 3,200,057

ELECTROPHORETIC COATING PROCESS Filed Dec. 27, 1960 2 Sheets-Sheet 2POWER SOURCE GEORGE EEBREWER G/LBER T L. BURNS/DE INVENTORS ATTORNEYS tobe coated serves as one electrode.

057 ELECTRUPHORET C COATING PROCESS Gilbert L. Burnside, Oak Park, andGeorge E. F. Brewer,

Wolverine Lake, Mich., assiguors to Ford Motor Company, Dearborn, Mich,a corporation of Delaware Filed Dec. 27, 196i Ser. No. 73,684 4 Qlairns.(Cl. 2tl4-18l) This invention relates to the formation ofelectrophoretic coatings and more particularly to an improved method ofcoating conducting surfaces of articles by electrophoretic techniques.

Present methods of painting articles having extended and irregularshaped surfaces include hand or automatic spray painting, electrostaticpainting, brushing, or dipping. Electrophoretic techniques have beenapplied for coating articles having a relatively small surface area butthese techniques have been restricted to the applicabution of paint ordue to surface blemishes.

In the past, it has also been rather difficult to obtain a relativelyuniform film of paint over the entire surface of an article. Surfaceswhich are not readily accessible to the application of paint have notbeen satisfactorily coated. Recessed areas, such as rocker panels ofautomobiles, have only been coated at a great expense due to therequirements of special painting techniques or special hand laboroperations to obtain a minimal protective coating against corrosion.'Dipping the article in a paint bath to obtain a coating of its recessedsurfaces has not proven satisfactory since the evaporation of solventsfrom the paint film with subsequent refluxing has resulted in washingoff a portion of the adhered paint coating. 7

This invention pertains to an improved method of electrophoreticallycoating articles which will provide a uniform coat of paint on all itssurfaces.

Electrophoresis has been defined as the motion of charged particlesthrough a suspending medium under the influence of an applied electricalfield. Electrophoretic coating may be carried out in any conventionalelectrolytic cell. The conducting surface of the article When thearticle to be coated is submerged in an electrolytic solution, anelectrical circuit is completed to a second electrode to initiatedeposition of emulsified colloidal particles suspended in theelectrolytic solution. In an electrical field, these colloidal particlesin suspension move either toward the negative or the positive electrodeshowing that the dispersed particles carry either a positive or anegative charge. Upon contact with an electrode, the colloidal particleslose their electrical charges, thereby breaking the emulsion anddepositing as a coating on the electrode.

Electrophoretic deposition of composition coatings is old in the art.

by submerging it in a bath of finely divided insulating organicparticles in a liquid medium. When a potential is applied between theanode and a cathode, which is United States Patent 2,307,018 disclosesone method of forming an insulating coating on an anode 3,29%,057Patented Aug. 10, .1965

"ice

ishing Magazine, volume 42, page 313 by Edward J. Roehl in an articleentitled A Discussion on Electrophoretic Finishing.

The electrolyte solution utilized in the electrophoretic coating processof this invention comprises emulsified paint particles in a colloidalstate dispersed in a suspended conducting liquid medium.

The paint emulsion particles are prepared so that each dispersed paintdroplet when suspended in an aqueous bath is uniform in composition andconsists of all film forming materials that are required for a practicalpaint coating. Each emulsion paint droplet is charged with either apositive or negative electrical charge so that they will migrate in anelectrical field that is deposited on an electrode. An exampleillustrating the formulation and preparation of one paint composition tobe utilized for electrophoretic coating is hereinafter described.

Current electrophoretic coatings of paint film have not provensatisfactory due to the nature of the electrophoretic coating process.Charged colloidal particles suspended in the electrolyte solutionmigrate towards the article being coated if the surface of the articlehas an electrical charge opposite in polarity .to the charge of thecolloidal material. When the charged particles come into contact withthe oppositely charged surface, they lose their electrical charge anddeposit as a coating. The deposition of this coating is irreversible inthe sense that the deposited particles lose their tendency to returninto colloidal dispersion. Since some of the dispersed colloidalmaterials in the solution have migrated towards the article being coatedwhile an electrical field is applied, a relatively high concentration ofsuch colloidal particles is present near the surface of the submergedarticle being coated. Upon the removal of the article from the solution,many of the colloidalparticles, which have not been deposited but remainin the vicinity of the submerged article, adhere to the alreadydeposited film. Upon the drying or curing of the deposited paint film,the nondeposited colloidal material adhering to the surface will bringabout an uneven coating which is very unsightly.

Rin sing of the excess undeposited colloidal material has resulted indamaging the deposited paint film and therefore, has not provensatisfactory for removing the excess colloidal material. Also, duringthe rinsing operation large amounts of the undeposited colloidalmaterials are washed away with the rinsing solution.

The improvement of the electrophoretic coating process of this inventioncomprises the reversal of the direction of the electrical field after anarticle is electrophoretically coated by changing the polarity of theelectrical charge applied to the electrolytic cell during theelectrophoretic coating process, i.e. changing the direction of flow ofdirect electrical current between the electrodes of such cell. Uponreversal of the electrical field, the colloidal material which has beenpreviously migrating towards the surface of the article now migratesaway from the surface of the article. After a reverse field is appliedfor a limited time, the depletion of the colloidal material in thesolution in the immediate vicinity of the submerged article is achieved.The article can then be removed from the solution without having anundeposited colloidal material adhering to its coated surface.

In US. Patent 1,769,659 it has been disclosed that in theelect-rophoretic coating process the direction of the current maybereversed after each complete deposition to prevent pitting of theelectrodes. In this patent, the potential applied during the depositionand reversal is limited to a value below the decomposition potential oftap water, which ranges between 1.2 and 1.9 volts, to prevent theformation of hydrogen and oxygen at the electrodes.

The electrical potential applied in the process of this force themigration of colloidal particles away from the vicinity of the submergedarticle is considerably less than the potential required to deposit acoating on the article.

Experimentally, voltages ranging between 50 volts to 1,000 volts haveproven very satisfactory at a current density of .1 ampere per squarefoot to amperes per .square foot to electrophoretically deposit a paintcoating. Voltages of 5 volts to 1,000 volts have been effective indepleting the colloidal particles in the immediate vicinity of thesubmerged article to prevent the adherence of colloidal particles to thedeposited paint coat. The current density to be selected for theelectrophoretic coating process will depend on various parameters suchas the The rinse solution may be mixed with a detergent, such as soap,to reduce its surface tension in order to avoid any damage to thedeposited paint film.' Very little or no agitation is required since nomechanical action is necessary to remove the undepositcd excesscolloidal material.

In a second embodiment of our invention, an article 11 iselectrophoretically coated in the coating bath 12 contained in the tank13 similar to the first embodiment as illustrated in FIGURE 2a. Afterthe article 11 is coated, it is not lifted from. the bath 12 but remainssubmerged in the same tank 13 while the direction of the electricalfield between the article 11 and the electrode 14 is reversed asselected voltage and the desired thickness of the coating, i

the conductivityand composition of the coating bath and the timeallotted for the formation of the desired coating, and the distancebetween the electrodes. Gaseous products given =ofi at the electrodesare of no serious concern in the process of electrophoretic coating ofpaint films of this invention since the paint composition is formulatedto effect a continuous film when polymerized.

An object of this invention is to provide for an improvedelectrophoretic coating process which will result in an even andunblemished paint film, thereby, improving the surface finish of thearticle being coated.

A further object of this invention is the elimination of the rinsingoperation which may be required after an article is electrophoreticallycoated.

A further object of this invention is the reduction of wastage of paintsince the undepo'sited colloidal material will substantially remain inthe aqueous coating bath where it can be used further for the coating ofadditional articles.

Other objects and advantages of this invention will become more apparentfrom the following description to be read in conjunction with theaccompanying drawings in which:

FIGURE 1 is a schematic drawing depicting the apparatus of the firstembodiment of this invention;

FIGURE 2a is :a schematic diagram showing the first phase ofelectrophoret-ically coating an article utilizing the equipment of thesecond embodiment of this invention;

FIGURE 21) is a schematic diagram illustrating the rinsing operationwhich is the second phase of the second embodiment of this invention;

FIGURE 3 is a schematic drawing of the third embodiment of thisinvention illustrating the electrophoretic coating of articles suspendedfrom a conveyor; and,

FIGURE 4 is a schematic drawing depicting the fourth embodiment of thisinvention showing a modified apparatus of the third embodimentillustrated in FIGURE 3.

In the first embodiment of our invention, an electrically groundedarticle 11 is electrophoretically coated by submerging it in a coatingbath 12 contained in an insulated tank 13, followed by the applicationof a negative electrical charge to an electrode 141 which is alsosubmerged in the coating bath 12. After the article 11 is electrophoretica'lly coated, it is removed tfrom the coating bath 12 andsubmerged in a rinse solution 15 contained in a second tank 16 which isgrounded.

During the eleotrophoretic coating operation, the negative electricalcharge is applied to the electrode 14 from the negative terminal of apower source 17. During the rinsing operation, the negative electricalcharge is directly applied to the article 11. When the electrical chargeis applied to the article 11 while it is submerged in the rinse solution15, undeposited excessive colloidal material which has been adhering tothe deposited film coat migrates away from the immediate vicinity of thesubmerged article 11.

shown in FIGURE 2b. The reversal of the electrical field is achieved byconnecting the article 11 to the negative terminal of the power source17 and electrically grounding the electrode. This reversal of theelectrical field will suit in the migration of the undeposited colloidalmaterial in the immediate vicinity of the submerged article 11 away fromits coated surface. The article 11 is then lifted from the coating bath12-the resulting surface film being uniform and relatively free fromundeposited material.

In a third embodiment of this invention, .as illustrated in FIGURE 3,the articles 11 to be coated are transported by a conveyor 18 to aninsulated tank 13 containing the electrophoretic coating bath 12. Thearticles 11 are hung from the conveyor 18 by a paint hook 19 which hasan electrically insulated portion 20 to isolate the article'11 fromground. A contact plate 21 is attached to the paint book 19 below itsinsulated portion 20. Article 11a, about to be coated, is grounded whenthe contact plate 21 makes an electrical connection with a grounded busbar 22. Article 11b, already coated, after having been su'bmerged in thecoating bath 12, becomes negatively charged when the contact plate 21contacts a second bus-bar 23 which is connected to the negativeterminalof the power source 17.

Colloidal paint particles in the coating bath 12 are repelled from the.vicinity of the article 11b which serves as a cathode of theelectrolytic cell. The chalged particles are propelled towards thearticle 11a which is grounded and are irreversibly deposited on itssurface. Articles 11a and 11b remain submerged in the coating bath 12until a sufficient paint film is deposited on the surface of article11a. Article 11b becomes a cathodeafter being coated by beingelectrically connected to the negative terminal of the power soure 17through the contact plate 21 and the bus bar 23. The uncoated article11a, which is conveyed to the tank 13, becomes submerged in the coatingbath 12 where it is grounded by the bus bar 22 to become the anode ofthe electrolytic cell.

Upon the removal of the coated article 11b from the coating bath 12,substantially no excess undeposited material will adhere to its coatedsurface due to the depletion of the colloidal material in the vicinityof the submerged coated article 11b during the coating of article 11a.The coated article 1112 is then conveyed to an oven 24 to cure thedeposited paint film. To keep the colloidal material uniformly dispersedin the coating bath 12, a mixer 25 is placed into the tank 13 toslightly agitate the coating bath 12. V g

This embodiment of our invention readily lends itself to a continuoushigh volume coating of articles by electrophoretic means which willresult in a high quality paint film without requiring a rinsingoperation. I

A further embodiment of this invention is illustrated in FIGURE 4. Asecond electrode 26 having a large surface area is placed into thecoating bath 12 contained in the tank 13 and is then electricallyconnected to the negative terminal of the power source 17. Upon theapplication of the electrical field, charged paint particles willmigrate from the vicinity of the coated article 11b and from theelectrode 26 to deposit a film on the electrically grounded article 11a.

The potential required to sufficiently propel particles away from thecoated surface 1112 does not have to be as great as that required todeposit charged particles on an uncoated surface 11a. By utilizing thesecond electrode 26 as a cathode, the required total potential betweenthe coated article 11b and the uncoated article 11a may be reduced sincethe coated article 11b, serving as the only cathode in the embodimentillustrated in FIGURE 3, has a relatively high resistance due to itsbuilt up insulating paint coat. Otherwise, a relatively high potentialwould be necessary to deposit the paint particles on the uncoatedarticle 11a. I

It is to be understood that for certain formulations of the coatingbath, in which the dispersed colloidal materials are positivelycharged,deposition can also be initiated on the surface of an article when thearticle serves as a cathode of the electrolytic cell.

One red primer which may be electrophoretically deposited as a primecoating on an electrically grounded steel panel is formulated asfollows:

A film forming material consisting of a styrene-allyl alcohol copolymer63 percent by Weight is mixed with linseed fatty acid 37 percent byweight. This mixture is esterified at 500 F. to an Acid Number of 5 anda maximum viscosity of 2.5 poises measured when the copolymer is reducedto 60 percent nonvolatile with xylene. A portion of this film formingmixture 51 percent by weight is intimately blended in a roller mill withred oxide pigment 45 percent by Weight and linseed fatty acid 4 percentby weight. This blend 38 percent by weight is then let down by furtherblending it with an additional portion of the film forming material ofthe above composition 50 percent by weight, malamine formaldehyde 11.8percent by weight, and cobalt naphthanate .2 percent by weight.

This pain-t mixture is then emulsified by the addition of an emulsifyingagent which consists of chemically pure concentrated ammonium hydroxide(28 percent strength) 3.5 percent by weight and demineralized water 96.5percent by weight. This diluted ammonium hydroxide solution is graduallyadded until the so-called inversion point is reached at which time theviscosity of the paint mixture drops suddenly after reaching a highpoint. The balance of the diluted ammonium hydroxide is then added. Theresulting emulsion is then further refined in a colloid mill until amore stable emulsion is reached.

The paint composition of the above example is then utilized to preparean aqueous coating bath for electrophoretically coating a metallicarticle. One example of the electrophoretic coating process utilizingthis paint composition is as follows:

Example 1 Two parts of the automotive primer paint composition describedabove is thoroughly mixed with 6 parts of water and A; part ofconcentrated ammonia to form a colloidal dispersion to be used as acoating bath. This coating bath has a specific resistance ofapproximately 5.0 10 ohm centimeter.

The metallic article to be coated is immersed in the bath andelectrically grounded. A negative charge is applied to an electrode alsoimmersed in the bath as described in the second embodiment of thisinvention illustrated in FIGURE 2a. This negative charge is graduallyincreased until the potential difference between the electrode and thearticle reaches 150 volts in an approximately 20 second time period. Theelectrical potential is then maintained at 150 volts for an additional40 seconds in order to obtain a uniform paint film of approximately 1mil thickness on the surface of the article after the paint film iscured. The specific resistivity of the uncured deposited paint film isapproximately 30x10 ohm centimeter, and the final current densitymeasures approximately 1.7 amperes per square foot.

After the desired film thickness is achieved, the article is lifted fromthe electrophoretic coating bath and immersed in a conductive rinse bathcontaining a grounded electrode as illustrated in FIGURE 2b. While thearticle is being rinsed, an electrical potential of approximately 45volts at a current density of /2 ampere per square foot is applieddirectly to the coated article for about a 30 second period. After therinsing operation, the article is removed from the bath and baked in anoven at a temperature of approximately 350 F. for 10 minutes to cure theelectrophoretically deposited prime coating.

Example 2 One part of the automotive primer paint composition heretoforedescribed is mixed with 6 parts of water and part of concentratedammonia and V part of a soap detergent to form a colloidal dispersion tobeused as a coating bath. The'specific resistance of this coating bathis approximately 1.0 10 ohm centimeter. The metallic article to be primecoated is immersed in the coating bath and electrically grounded. Anelectrode, which is also immersed in the bath, has an electrical chargeapplied which is gradually increased to 200 volts in 30. seconds; Thispotential is maintained between the electrode and the article forapproximately an additional 60 seconds in order to obtain a paint filmof approximately 1 mil thickness on the surface of the article after thefilm is cured. The specific resistivity of this paint film on the coatedarticle measures approximately 2.1 X 10" ohm centimeter, and the finalcurrent density measures approximately 3.5 amperes per square foot.

After the desired film thick-ness is achieved, the electrode is groundedand an electrical potential of 200 volts is applied directly to thecoated article for 20 seconds to remove the excess colloidal materialfrom the vicinity of the immersed coated article. The article is thenremoved from the bath and baked in an oven at an approximate temperatureof 375 F. for about 10 minutes to cure the deposited prime paint coatingon the surface of the metallic article.

The power source 17 utilized in Examples 1 and 2 is a rectifier having arange of 0 and 1,000 volts and 0 and amperes.

We claim:

1. In a method of applying a coating to a conducting surface of anarticle comprising the steps of causing the surface of said article tocontact an aqueous bath having organic film-forming material dispersedtherein, said article serving as a first electrode, causing a secondelectrode to contact said aqueous bath, and causing a direct electriccurrent to flow between said first and second electrodes and throughsaid aqueous bath at an electrical potential substantially above thevalue at which the electrolysis of water occurs until a coating of saidfilm-forming material is electrically deposited upon said article fromsaid aqueous bath, the step of causing the direction of flow of thedirect electric current between said article and said bath to bereversed for a time sufiicient to remove from the surface of saidelectrically deposited coating film-forming material from said bath thathas not been electrically deposited.

2. In a method of applying a coating to a metallic article comprisingthe steps of immersing said article in an aqueous bath having chargedparticles of organic coating material dispersed therein, said articleserving as a first electrode, causing a second electrode to contact saidaqueous bath, and causing a direct electric current to flow between saidfirst and second electrodes and through said aqueous bath at an electricpotential of between 50 and 1000 volts until a Water insoluble coatingof said coating material is electrophoretically deposited upon saidarticle from said aqueous bath, the step of causing the direction offiow of direct electric current between said article and said bath to bereversed for a time sufiicient to remove charged particles of coatingmaterial from said water insoluble coating.

3. In a method of applying paint to a metallic article comprising the"steps of causing the surface of-said article to be immersed in anaqueous bath having a paint formulation comprising a film-formingorganic resin dispersed therein, said article serving as a firstelectrode, causing a second electrode to contact said aqueous bath, andcansing a direct electric current to flow between'said first and saidsecond electrodes and through said aqueous bath at an electric potentialsubstantially above the value at which the electrolysis of water occursuntil a water-insoluble film of paint is'electrically deposited uponsaid article from said aqueous bath, the steps of causing the directionof -flow of direct electric current between said article and said bath.to be reversed for a timesufficient to remove adhered paint particlesfrom the electrically deposited paint film.

.4. In a method of applying paint to a metallic article comprising thesteps of causing the surface of said article to be immersed in anaqueous bath having charged parti cles ofa paint formulation comprisinga film-forming organic resin dispersed ther'ein, said article serving asa first electrode, causing a secondselectrode to contact said aqueousbath, and causing a direct electric current to fiow between said firstand said second electrodes and through said aqueous bath at an electricpotential in'the range of about 50 to about 1000 volts until a waterinsoluble film of paint is electrophoretically deposited upon saidarticle from said aqueous bath, the step of causing the direction offlow of direct electric current between said article and said bath to bereversed for a time sufiicient to remove charged paint particles fromthe electrophoretically deposited paint-film at a voltage in the rangeof about 5 to about 1000 volts. i

References Cited by the Examiner UNITED STATES PATENTS WINSTON A.DOUGLAS, Primary Examiner. JOHN R. SPECK, MURRAY TILLMAN, Examiners.

1. IN A METHOD OF APPLYING A COATING TO A CONDUCTING SURFACE OF ANARTICLE COMPRISING THE STEPS OF CAUSING THE SURFACE OF SAID ARTICLE TOCONTACT AN AQUEOUS BATH HAVING ORGANIC FILM-FORMING MATERIAL DISPERSEDTHEREIN, SAID ARTICLE SERVING AS A FIRST ELECTRODE, CAUSING A SECONDELECTRODE TO CONTACT SAID AQUEOUS BATH, AND CAUSING A DIRECT ELECTRICCURRENT TO FLOW BETWEEN SAID FIRST AND SECOND ELECTRODES AND THROUGHSAID AQUEOUS BATH AT AN ELETRICAL POTENTIAL SUBSTANTIALLY ABOVE THEVALUE OF AT WHICH THE ELECTROLYSIS OF WATER OCCURS UNTIL A COATING OFSAID FILM-FORMING MATERIAL IS ELECTRICALLY DEPOSITED UPON SAID ARTICLEFROM SAID AQUEOUS BATH, THE STEP OF CAUSING THE DIRECTION OF FLOW OF THEDIRECT ELECTRIC CURRENT BETWEEN SAID ARTICLE AND SAID BATH TO BEREVERSED FOR A TIME SUFFICIENT TO REMOVE FROM THE SURFACE OF SAIDELECTRICALLY DEPOSITED COATING FILM-FORMING